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Two prototype experiments allowing a simultaneous mode II and mode IIIfatigue crack propagation in a single specimen are described and the differences inrelated to growth are discussed. The cylindrical specimens made of austenitic andferritic steels with circumferential V-notch were prepared. The specially manufacturedloading setups enabled to assure a pure remote shear mode II at both the top and thebottom sites of the specimen, whereas a pure mode III operated at front and back sites.Differences between the mechanisms of crack propagation were assessed by means ofthe 3D fractographical analysis based on the stereophotogrammetry in SEM. The stressintensity factors KII and KIII were determined by a numerical method based on theANSYS code and compared with asymptotically computed values of KIII. The thresholdvalues ΔKIIth (R = 0.1) were found to be of 1.2 MPam1/2 in ferrite and of 3.5 MPam1/2 inaustenite, whereas ΔKIIIth ≈ 2.0 .in ferrite and ΔKIIIth ≈ 4.7 MPam1/2 in austenite. In bothsteels, the near-threshold crack growth rate under the mode II was found to be muchhigher than that under the mode III.